This invention relates generally to measuring coating thickness and, more specifically to measuring coating thickness on metal surfaces.
Sol-gel coatings are created upon metallic substrates for a great variety of purposes. For example, titanium foil is frequently covered with a sol-gel coating when forming laminate structures where the sol-gel promotes adhesion of the various layers in the structure. Sol-gel coatings are generally made using wet chemistry methods where the liquid sol-gel mixture is created and optimized prior to applying it to the metal substrate. The sol-gel is typically applied by spraying the sol-gel on the metal surface. The sol-gel is then cured at elevated temperature to give the desired properties.
Generally, a uniform coating thickness or a coating thickness within an acceptable range is desired. However, determining uniformity of the coating thickness or quantifying the coating thickness relative to a desired range may be difficult. Current coating thickness testing methods are destructive and therefore cannot be used with final production products. They are also time consuming, environmentally unfriendly, and disruptive to large scale production processes. Sol-gel coating thickness is sometimes specified for some applications and there is no simple non-destructive for measurement of sol-gel thickness currently known in the art.
Current coating thickness testing known in the art is performed by sputtering away the sol-gel coating using depth profiling Auger spectroscopy or Glow Discharge Optical Emission Spectroscopy (GD-OES). These methods gradually remove the sol-gel coating and can determine where the sol-gel ends and the metal substrate begins. Sol-gel thickness is measured by calibrating the sol-gel removal rate and measuring the time required to remove the sol-gel layer. In addition, the currently known testing methods make very small measured spots for coating thickness on the sample. As such, the currently known testing processes require many time-consuming measured spots to determine coating thickness variations over an area.
Therefore, there exists an unmet need in the art for a nondestructive method of determining sol-gel coating thickness on a metallic substrate.
The present invention provides a nondestructive method for efficiently determining thickness of a sol-gel coating formed upon a metallic substrate without sputtering away the sol-gel coating. The invention may be employed in an in-line production facility or may be used intermittently as desired. The process may be used to provide a quantitative measurement, such as actual coating thickness, or a qualitative measurement, such as a go or no-go result.
According to one embodiment of the invention, a non-destructive method is provided for determining the thickness of a sol-gel coating on a metallic substrate. A value of infrared energy reflected from the metallic substrate without the sol-gel coating is determined. A value of infrared energy reflected from the metallic substrate with the sol-gel coating is determined. A value of infrared energy absorbed in the sol-gel coating is determined, and a value of the infrared energy absorbed in the sol-gel coating is correlated to a thickness of the sol-gel coating.
According to an aspect of the invention, one embodiment of the invention includes transmitting an infrared beam having a predetermined wavelength through a sol-gel coating on a metallic substrate at a predetermined incident beam angle. The transmitted beam has a cross-sectioned area to produce a predetermined spot size on a surface of the sol-gel coating. The infrared beam is reflected off the metallic substrate to form a reflected beam and the reflected beam is filtered to a predetermined wavelength band, if desired, and detected. The infrared energy of the reflected beam is compared with a predetermined value of infrared energy reflected off the metallic substrate without the sol-gel coating to determine an absorbance value for the sol-gel coating. The absorbance value for the sol-gel coating is correlated to a thickness of the sol-gel coating.